Before the 2016 election, these fact-checker sites were mainly used to double-check what politicians said in major speeches and debates, or to analyze urban legends. But with so many supposed news posts spreading across social media and the era of Fake News, these sites are quite useful and traffic is much heavier.
For major speeches, most major news outlets will offer an annotated, fact-checked, version of the transcript. I've seen them in the New York Times, Washington Post, and NPR.
These sites have no overt political bias. Most spring from newspapers double-checking political speeches.